Structure and Development of a Horny Spov(je. 7 1 



from the number seen in si single transverse section, it would 

 a])pear that each of the large epithelial cells may have a 

 liuiidred or more sac-shaped ectodermal embiyonic cells 

 hanging from its lower surface. 



'J'he unusual length of time during which the embrj^o 

 remains within the mother sponge, and the great size to 

 which it attains, necessitate some special arrangement 

 whereby it can be nourished. The peculiar relations of the 

 ectodermal cells of the embryo to the investing epithelium, 

 and the very unusual character of the latter, cause me to 

 believe that the investing epithelium has for its function the 

 nutrition of the embryo, and that this is effected by the 

 absorption of nutriment through the necks of the ectodermal 

 cells. 



Sometimes, however, the ectodermal cells exhibit no 

 prolongations of the neck, but are smoothly rounded off at the 

 free end, and such cells may form a continuous layer over a 

 considerable area. 



The entire mass oi the embr3'o, Avithin tlie ectodermal 

 la3'er, is made up of a clear, jelly-like matrix, in which 

 immense numbers of large, amoeboid wandering cells are 

 imbedded. These cells appear somewhat larger than the 

 ectodermal cells, but there is ver}^ strong reason for believing 

 that they are simply ectodermal cells which have left their 

 ])Iace in the outer layer and, becoming amoeboid, wandered 

 into the central jelly. Between the large amoeboid cells very 

 delicate branching stellate cells may sometimes be seen. 

 The amoeboid cells may put out pseudopodia in all directions, 

 but often tliey appear to be radially elongated, and more or 

 less bi-polar. I think my sections show conclusively that 

 the amoeboid cells are derived from the ectodermal layer. 

 They agree at tirst in all essentials with the cells of the 

 latter, and in those parts where the ectodeiinal cells have the 

 clearer, outer end of the neck evenly rounded off — and thus 

 present a characteiistic feature — a precisely similar clear 

 rounded neck may often be seen in the cells immediately 

 beneath the ectoderm. 



The amoeboid cells are from the first highly granular, and 

 at what I believe to be an early stage in the proceedings 

 each one has a sy)herical nucleus, resembling that which 

 occurs in the ectodermal cells. Sometimes the amoeboid 

 cells lying near the outside of the emlnyo have two or three 

 nuclei, and very rarely also even the ectodermal cells appear 

 to have two nuclei. At a later stage, the entire cell is seen to 



